The present invention relates to an improvement in a conference telephone apparatus which includes a microphone and a loudspeaker and which has a voice switching circuit for eliminating howling.
More particularly, the present invention is concerned with a conference telephone apparatus which allows persons in a room to hold hands-free, mutual conversations in a conference call with persons in a remote room by means of microphones and loudspeakers. The two remote rooms are interconnected by a communication line.
In a hands-free conference telephone system of the type described, a voice switching circuit prevents howling from developing due to acoustic coupling between a microphone and a loudspeaker. The voice switching circuit compares the level of a transmitted speech signal picked up by the microphone with the level of a received speech signal sent from the other party. The switching circuit removes a loss from a communication channel associated with the one of the speech signals which has a level that is higher than the level in another channel. This removal of a loss sets up a normal gain (gain of an amplifier circuit necessary for ordinary communications), while simultaneously inserting a loss in a communication channel associated with the channel having a lower speech signal level. A comparator cicuit included in the voice switching circuit has rectified and smoothed versions of the transmitted and received signals.
The charging and discharging time constants of a rectifying and smoothing circuit have heretofore been predetermined with the following principle. It is essential that the buildup time of the voice switch be as short as possible in order to clip to a minimum the number of initial syllables of a speech signal which is to be interrupted. Hence, the charging time constant simply needs to be short. The discharging time constant, on the other hand, should be based on a relationship between the characteristics which prevent clipping the last syllables of a speech signal which is being interrupted and the changeover characteristics of the voice switching circuit from a transmitting state to a receiving state, and vice versa, during mutual communication with the other party, i.e. speech exchangeability. However, it has been a common practice to select a fixed discharging time constant at the sacrifice of speech exchangeability on the assumption that a reverberation time in a conference room is generally 0.5-0.6 second, so that the voice switch may be prevented from being erroneously changed from the receiving state to the transmitting state in response to a received speech signal which represents a voice signal which was amplified by a loudspeaker and picked up by a microphone.
In another known system, a level control is performed such that a predetermined loss or a predetermined gain is applied, for a given period of time, to a transmitting signal level and a receiving signal level which re fed to the comparator circuit, depending upon the state of the voice switch, so as to stably maintain the then existing condition of the voice switch.
In short, prior art conference telephone systems, with voice switching circuits, are commonly constructed with fixed parameters which only match assumed ranges of reverberation time in rooms and acoustic coupling between microphones and loudspeakers. This brings about a severe situation even when actual acoustic conditions in a room are better than the presumed ones. No improvement is attainable in speech exchangeability. Or, when room conditions are worse than the presumed ones, e.g., in a conference room whose reverberation time is longer than 0.6 second, for example, a "received speech blocking" occurs in which a received speech interrupts itself.